The present invention relates to a trigger switch capable of controlling the velocity of a motor mounted in an electric tool or the like.
A conventional trigger switch of this type is described in U.S.P. No. 3,543,120. A thyristor is used in this trigger switch for controlling the velocity of the motor. Since the thyristor generates a great amount of heat, the heat must be effectively radiated. Conventionally, the thyristor is embedded in a printed circuit board, and heat is radiated through the printed circuit board. However, the printed circuit board is heated by heat from the thyristor to a high temperature, resulting in an adverse effect in other circuit components. In particular, when a resistor is formed in the printed circuit board, the resistance of the resistor changes in accordance with the temperature of the printed circuit board. As a result, the velocity control for the motor becomes unstable.
In motor velocity control, when the stroke of the trigger is adjusted and a desired velocity is obtained while the trigger is withdrawn, a desired velocity can be easily obtained, thereby providing good operability. However, it is very difficult to realize a stroke adjusting mechanism having a rigid structure which withstands the trigger operation.